Diene polymer is used as, for example, a rubber component in a rubber composition. It is required in a rubber composition, particularly a rubber composition for a tire, to improve rolling resistance performance related to low fuel consumption, and wet performance that is grip performance on a wet road surface. However, rolling resistance performance and wet performance have the antinomic relationship, and it is not easy to improve those performances simultaneously. For example, a glass transition point is operated by changing an amount of styrene in a styrene-butadiene rubber, thereby wet performance can be improved. However, only the operation of a glass transition point deteriorates rolling resistance performance.
It is conventionally known to modify a diene polymer, and it is proposed to improve rolling resistance performance and wet performance by the operation of polarity by, for example, grafting onto a polymer chain, modification of a terminal, or addition of a functional group (for example, see PTLs 1 to 6 below). However, it is not easy to simultaneously improve rolling resistance performance and wet performance. Furthermore, there is no conventional art to easily introduce a functional group in a main chain structure, regardless of solution polymerization or emulsion polymerization.
PTL 7 below discloses a depolymerized natural rubber useful as an adhesive, a pressure-sensitive adhesive or the like. PTL 7 produces a liquid depolymerized natural rubber having a number average molecular weight of from 2,000 to 50,000 by air oxidation of a deproteinized natural rubber dissolved in an organic solvent in the presence of a metal catalyst to perform depolymerization. PTL 7 discloses that a main chain is decomposed by air oxidation to form a molecular chain having a carbonyl group at one terminal and a formyl group at other terminal, and the formyl group is then recombined by aldol condensation. However, in PTL 7, depolymerization is conducted in a solution of an organic solvent, and it is not disclosed that the recombination is performed by changing a system containing decomposed polymers from an acidic system to a basic system or from a basic system to an acidic system.